162 BELL SYSTEM TECHNICAL JOURNAL 



and conductance values. These shields must not, of course, be allowed 

 to act as short-circuited secondaries on the transformer which would 

 be the case if they linked conductively with the windings. Each is 

 therefore made in two parts similar to toroidal channels which upon 

 assembly have their overlapping inner circumferences insulated from 

 each other by means of thin mica laminations. Further details of the 

 construction will be evident from a study of Fig. 14. 



Fig. 14 — Shielded transformer 



The windings are, of course, proportioned so as to connect with a 

 reasonable degree of efficiency the associated impedances. For best 

 results two sets of transformers are used to cover the complete fre- 

 quency range, one from 500 to 5000 cycles and the other from 5000 

 to 50,000 cycles. 



Balancing Condensers arid Impedance Arm Balance. It has been 

 brought out previously that two adjustable capacitances are required, 

 one to effect the proper adjustment of the bridge capacitances to 

 ground and the other to balance the residual capacitances shunting one 

 of the impedance arms. Such capacitances are provided in the form 

 of adjustable air condensers each having a maximum value of about 

 500 mmf. The construction used is such as to give a high degree of 

 stability of capacitance combined with low conductance characteristics. 

 The arrangement of these condensers in relation to the other apparatus 

 is shown in Fig. 3. 



The effect on the accuracy of the bridge of the degree of balance 

 of the impedance arms obtained by means of the balancing condenser 

 Ch is determined as follows : 



Capacitance Shunting the Impedance Arms. The equations giving 

 the equivalent series inductance L' and resistance R' of a reactance of 



